arXiv subjects

Mutsuko Hatano

Publications and source records attributed to Mutsuko Hatano.

At least 19 recordsLinked to original sources

arXiv2026

Enhanced Emission and Two-Photon Interference of Lead-Vacancy Centers in Diamond Solid Immersion Lenses

A negatively charged lead-vacancy (PbV-) center in diamond is a novel quantum system which can be operated at a high temperature above 4 K owing to its large ground state splitting. To unlock the quantum properties of the PbV- center, the enhancement of the fluorescence intensity is a key issue. Here, we demonstrate enhanced emission from PbV- centers by a factor of approximately 10 using solid immersion lenses (SILs) fabricated on rough diamond surface caused by high-temperature anneal over 2000°C. Resonant excitation reveals the narrow emission close to the transform-limited linewidth. Furthermore, we demonstrate two-photon interference using one single PbV- center in an SIL. This enhanced emission and indistinguishability will lead to further development of the PbV- center towards quantum network node applications.

quant-ph↗

arXiv2026

Observation of individual vortex penetration in a coplanar superconducting resonator

We demonstrate the microwave signatures of individual Abrikosov-vortex penetration events in superconducting microwave resonators. $λ/4$ resonators with a narrowed region near the grounded end acting as a vortex trap were fabricated and studied using microwave transmission spectroscopy at millikelvin temperatures. Sharp stepwise drops in resonance frequency are detected as a function of increasing external magnetic field, attributed to the entry of individual Abrikosov vortices in the narrow region. This interpretation is confirmed by NV center magnetometry revealing discrete vortex entry events on increasing field and the absence of such drops in a device with a wider narrowed region. Our results establish a method to investigate and manipulate the states of Abrikosov vortices with microwaves access and readout vortex configurations using magnetic-field sweeps and microwave drive.

cond-mat.supr-con↗

arXiv2026

Wide-field magnetic imaging of shielding-current-driven vortex rearrangement under local heating using diamond quantum sensors

Understanding and controlling vortex motion in superconductors are important both for suppressing dissipation in superconducting devices and for device applications that exploit vortices. In this work, we quantitatively imaged the stray magnetic field distribution of vortices in an NbN thin film by wide-field magnetic imaging using a perfectly aligned diamond nitrogen-vacancy ensemble. By continuously measuring while stepwise varying the applied magnetic field under local laser heating, we captured a rearrangement of the vortex configuration in real space and in real time over more than 100 min. The observed vortex rearrangement is consistent with a reduction of the pinning force due to local laser heating and with the Lorentz force exerted by shielding currents induced by the field variation. These results provide insight into vortex dynamics and suggest potential applications, including vortex exclusion from sensitive regions of superconducting devices.

cond-mat.supr-con↗

arXiv2026

Coherent Dark State Formation of a Lead-Vacancy Spin Qubit in Diamond

A lead-vacancy (PbV) center in diamond exhibits coherent emission above the liquid helium temperature, making it highly attractive for quantum network applications. Here, we report the magneto-optical and spin properties of PbV centers in diamond. We record a spin lifetime of 12 ms at 7.5 K under large off-axis magnetic field. Furthermore, we observe formation of the coherent dark state by coherent population trapping and estimate a spin dephasing time of 177 ns at 6.5 K. This work demonstrates the outstanding thermal robustness of the PbV spin compared to other group-IV centers above 4 K.

quant-ph↗

arXiv2026

A Highly Sensitive Diamond NV Magnetometer Using Ramsey Interferometry with a Short Sensor-to-Sample Distance

In this study, we developed a diamond quantum magnetometer based on Ramsey interferometry with a short sensor-to-sample distance. Conventional biomagnetic sensors with ensemble nitrogen-vacancy centers using continuous-wave optically detected magnetic resonance and Ramsey methods typically rely on watt-level lasers to achieve high sensitivity, resulting in thermal issues. In contrast, by employing the light-trapping diamond waveguide technique in a high-pressure and high-temperature diamond sample treated with electron beam irradiation, we obtained a high photon conversion efficiency of 9.5%, enabling us to simultaneously achieve a high sensitivity of 2.93(7) pT/Hz^1/2 in the 100-400 Hz frequency range and a minimal temperature increase of only approximately 13 K at a low laser power of 210 mW. Using a dry phantom designed to mimic magnetoencephalography signals, we measured a weak magnetic field of 77.7(2) pT without signal averaging at a sensor-to-sample distance of 2.5 mm. This short-distance measurement prevents severe spatial signal attenuation, yielding a high signal-to-noise ratio. The development here is crucial for practical biomagnetic applications based on Ramsey interferometry.

quant-ph↗

arXiv2025

Visualization of Current-Driven Vortex Formation in High-$T_c$ Cuprate Superconductors

Type-II superconductors exhibit hysteretic behavior due to the presence of quantum vortices, and the order in which temperature and external field are varied plays a decisive role. Here we take current, rather than magnetic field, as the external drive. We image the magnetic field of a high-$T_c$ cuprate superconductor strip after cooling. We confirm that even in zero magnetic field, current-biased cooling nucleates vortices within the strip. With a small external magnetic field, the distribution is polarized opposite to the Lorentz-force direction. These behaviors follow from the self-consistent relation between current and local field in steady flux flow. Our findings show that current history is encoded as vortices. This reveals self-field effects that influence dc measurements and glassy transitions under drive.

cond-mat.supr-con↗

arXiv2025

Multichannel highly sensitive diamond quantum magnetometer

We demonstrate a highly sensitive real-time magnetometry method at two measurement points. This magnetometry method is based on the frequency-division multiplexing of continuous-wave optically detected magnetic resonance. We use two ensembles of nitrogen-vacancy (NV) centers separated by 3.6 mm to measure a magnetic field. A different bias field is applied to the two NV ensembles to resolve the resonance peak for each ensemble in the frequency space and enables the multiplexed magnetometry at the two points. The sensitivities achieved at the measurement points are $21~\mathrm{pT/\sqrt{Hz}}$ and $22~\mathrm{pT/\sqrt{Hz}}$. The proposed magnetometry method can be expanded to include more measurement points and shorter spacing. The capability of real-time measurement at numerous points with short spacing and high sensitivity is beneficial for various applications, including biomagnetic sensing, geophysical research, and material science.

quant-ph↗

arXiv2025

Optical charge state manipulation of lead-vacancy centers in diamond

Group-IV vacancy centers in diamond exhibit excellent optical and spin coherence properties, making them highly promising and scalable spin qubit candidates. Since only specific charge states are magneto-optically active, control over the charge state is fundamental for quantum applications. Here, we realize the charge state control of lead-vacancy centers (PbV) through multi-color laser irradiation. We achieve tunable population manipulation of the negatively charged state from 0 to 89%, paving the way for spin control of the negatively charged PbV center. Furthermore, through analysis of charge state dynamics, we propose a charge cycle between the neutral and negatively charged states, indicating a possible pathway to the neutral PbV center with a spin-1 system.

quant-ph↗

arXiv2025

Current comparator for both AC and DC ratio measurements with 10-8-level accuracy

Accurate measurements of alternating current (AC) and direct current(DC) ratios are fundamental to electric power metrology. However, conventional current comparators for AC and DC typically rely on distinct technologies-electromagnetic induction for AC and superconducting quantum interference devices for DC. This technological divide leads to a fragmented and complex traceability system. Bridging this gap is critical for developing unified current standards that meet the demands of emerging power technologies. In this work, we present a compact, room-temperature AC/DC current comparator that integrates a diamond-based magnetometer using nitrogen-vacancy centers. The device achieves an accuracy of 10-8 for both AC and DC signals and supports a system bandwidth up to 300 Hz, without the need for cryogenics. It surpasses the performance of typical AC comparators, offering ten-fold higher accuracy, and matches that of state-of-the-art DC comparators. This unified, cryogenics-free solution not only enhances precision and versatility but also expands the applicability of the system to DC resistance bridges in quantum electrical standards.

quant-ph↗

arXiv2025

Narrow Inhomogeneous Distribution and Charge State Stabilization of Lead-Vacancy Centers in Diamond

Lead-vacancy (PbV) centers in diamond with a large ground state splitting are expected to be a building block of quantum network nodes. Due to the heaviness of the Pb atom, it is challenging to fabricate high-quality PbV centers with a narrow inhomogeneous distribution and stable charge state. In this study, for the formation of the PbV centers, high temperature anneal up to 2300°C is performed after Pb ion implantation. At a lower temperature of 1800°C, the PbV centers show a large inhomogeneous distribution and spectral diffusion, while higher temperatures of 2200-2300°C leads to narrow inhomogeneous distributions with standard deviations of ~5 GHz. The charge state transition of the PbV centers formed at 2200°C occurs by capturing photo-carriers generated from surrounding defects under 532 nm laser irradiation. Finally, multiple stable PbV centers with nearly identical photon frequencies are obtained, which is essential for applications in quantum information processing.

quant-ph↗

arXiv2024

Performance Evaluation of a Diamond Quantum Magnetometer for Biomagnetic Sensing: A Phantom Study

We employ a dry-type phantom to evaluate the performance of a diamond quantum magnetometer with a high sensitivity of about $6~\mathrm{pT/\sqrt{Hz}}$ from the viewpoint of practical measurement in biomagnetic sensing. The dry phantom is supposed to represent an equivalent current dipole (ECD) generated by brain activity, emulating an encephalomagnetic field. The spatial resolution of the magnetometer is evaluated to be sufficiently higher than the length of the variation in the encephalomagnetic field distribution. The minimum detectable ECD moment is evaluated to be 0.2 nA m by averaging about 8000 measurements for a standoff distance of 2.4 mm from the ECD. We also discuss the feasibility of detecting an ECD in the measurement of an encephalomagnetic field in humans. We conclude that it is feasible to detect an encephalomagnetic field from a shallow cortex area such as the primary somatosensory cortex.

physics.ins-det↗

arXiv2024

Charge state transition of spectrally stabilized tin-vacancy centers in diamond

Solid-state quantum emitters are an important platform for quantum information processing. The fabrication of the emitters with stable photon frequency and narrow linewidth is a fundamental issue, and it is essential to understand optical conditions under which the emitter keeps a bright charge state or transitions to a dark state. For these purposes, in this study, we investigate the spectral stability and charge state transition of tin-vacancy (SnV) centers in diamond. The photoluminescence excitation spectra of multiple SnV centers are basically stable over time with nearly transform-limited linewidths under resonant excitation, while simultaneous irradiation of resonant and non-resonant lasers makes spectra from the SnV centers unstable. We find that the instability occurs due to the charge state transition to a dark state. The charge state transition rates are quantitatively investigated depending on the laser powers. Lastly, with first-principle calculations, we model the charge state transition of the SnV center under the laser irradiation.

quant-ph↗

arXiv2023

Wide-field quantitative magnetic imaging of superconducting vortices using perfectly aligned quantum sensors

Various techniques have been applied to visualize superconducting vortices, providing clues to their electromagnetic response. Here, we present a wide-field, quantitative imaging of the stray field of the vortices in a superconducting thin film using perfectly aligned diamond quantum sensors. Our analysis, which mitigates the influence of the sensor inhomogeneities, visualizes the magnetic flux of single vortices in YBa$_2$Cu$_3$O$_{7-δ}$ with an accuracy of $\pm10~\%$. The obtained vortex shape is consistent with the theoretical model, and penetration depth and its temperature dependence agree with previous studies, proving our technique's accuracy and broad applicability. This wide-field imaging, which in principle works even under extreme conditions, allows the characterization of various superconductors.

cond-mat.supr-con↗

arXiv2023

Fast coherent control of nitrogen-14 spins associated with nitrogen-vacancy centers in diamonds using dynamical decoupling

A nitrogen-vacancy (NV) center in a diamond enables the access to an electron spin, which is expected to present highly sensitive quantum sensors. Although exploiting a nitrogen nuclear spin improves the sensitivity, manipulating it using a resonant pulse requires a long gate time owing to its small gyromagnetic ratio. Another technique to control nuclear spins is a conditional rotation gate based on dynamical decoupling, which is faster but unavailable for nitrogen spins owing to the lack of transverse hyperfine coupling with the electron spin. In this study, we generated effective transverse coupling by applying a weak off-axis magnetic field. An effective coupling depends on the off-axis field; the conditional rotation gate on the nitrogen-14 spins of an NV center was demonstrated within 4.2 μs under an 1.8% off-axis field and a longitudinal field of approximately 280 mT. We estimated that a population transfer from the electron to nitrogen spins can be implemented with 8.7 μs. Our method is applicable to an ensemble of NV centers, in addition to a single NV center.

quant-ph↗

arXiv2023

Transform-Limited Photon Emission From a Lead-Vacancy Center in Diamond Above 10 K

Transform-limited photon emission from quantum emitters is essential for high-fidelity entanglement generation. In this study, we report the coherent optical property of a single negatively-charged lead-vacancy (PbV) center in diamond. Photoluminescence excitation measurements reveal stable fluorescence with a linewidth of 39 MHz at 6 K, close to the transform-limit estimated from the lifetime measurement. We observe four orders of magnitude different linewidths of the two zero-phonon-lines, and find that that the phonon-induced relaxation in the ground state contributes to this huge difference in the linewidth. Due to the suppressed phonon absorption in the PbV center, we observe nearly transform-limited photon emission up to 16 K, demonstrating its high temperature robustness compared to other color centers in diamond.

quant-ph↗

arXiv2022

Identical Photons from Multiple Tin-Vacancy Centers in Diamond

We report the narrow inhomogeneous distribution of the zero-phonon line from tin-vacancy (SnV) centers in diamond and the overlap of spectra from multiple separated SnV centers. Photoluminescence excitation spectroscopy measurements at a cryogenic temperature showed that SnV centers exhibit stable fluorescence and linewidths close to the Fourier transform-limited linewidth. The inhomogeneous distribution was as low as ~4 GHz, which enabled the observation of Sn isotope-dependent resonant frequencies. Owing to the narrow inhomogeneous distribution, we observed multiple SnV centers showing identical photons with almost the same wavelength and linewidth. Identical SnV centers were also observed even in different diamond samples, confirming the reliable fabrication of the high-quality SnV centers.

quant-ph↗

arXiv2022

Coherence of a charge stabilised tin-vacancy spin in diamond

Quantum information processing (QIP) with solid state spin qubits strongly depends on the efficient initialisation of the qubit's desired charge state. While the negatively charged tin-vacancy ($\text{SnV}^{-}$) centre in diamond has emerged as an excellent platform for realising QIP protocols due to long spin coherence times at liquid helium temperature and lifetime limited optical transitions, its usefulness is severely limited by termination of the fluorescence under resonant excitation [1,2,3]. Here, we unveil the underlying charge cycle of group IV-vacancy (G4V) centres and exploit it to demonstrate highly efficient initialisation of the desired negative charge state of single SnV centres while preserving long term stable optical resonances. We furthermore all-optically probe the coherence of the ground state spins by means of coherent population trapping and find a spin dephasing time of 5$μ$s. Additionally, we demonstrate proof-of-principle single shot spin state readout without the necessity of a magnetic field aligned to the symmetry axis of the defect.

quant-ph↗

arXiv2021

Low-Temperature Spectroscopic Investigation of Lead-Vacancy Centers in Diamond Fabricated by High-Pressure and High-Temperature Treatment

We report the optical observation of lead-vacancy (PbV) centers in diamond fabricated by Pb ion implantation and subsequent high-temperature annealing (2100 °C) under high pressure (7.7 GPa). Their optical properties were characterized by photoluminescence at varying temperatures down to 5.7 K. We observed intense emission peaks at 550 and 554 nm with a large splitting of approximately 3900 GHz. The two lines are thought to correspond to the zero phonon line (ZPL) of PbV centers with split ground and excited states. A cubic trend of the ZPL width was observed while varying temperature. We performed polarization measurements of the two lines in a single PbV center, showing nearly orthogonal dipole polarizations. These optical measurements strongly indicate that the PbV center possesses D3d symmetry in the diamond lattice. The observed large ground state splitting significantly suppresses the phonon-mediated transition, which causes decoherence of the electron spin state of the group IV color centers in diamond, expecting a long spin coherence time at a temperature of approximately 8 K.

quant-ph↗